Hide internal checkpoint docs from `$db/_local_docs` response

[nickva]

Currently $db/_local_docs shows internal (system) checkpoint documents for purge and internal replicator. Users are often confused by those and may try to remove them. Deleting them may or may not succeed, because they are created "below" the cluster document sharding layer. If it does succeed, it could reset the internal replication progress or, in case of the purge ones, would simply be re-created by the views next time they are opened.

It would be helpful to hide them from the _local_docs output. For debugging it may be nice to allow passing in a parameter to view them ?include_system=true or similar.